1. Field of the Invention
The present invention relates to a near field communication system. In particular, the present invention relates to a near field communication system ensuring that the communication is stable regardless of the location of the antenna of a portable device.
2. Description of the Related Art
Near field communication (NFC) is a wireless and contactless communication technology, and it is widely used in many apparatus and devices; such as smart card, smart phone or tablet computer. The NFC technology uses magnetic induction to transfer data from one antenna to another antenna over a very short distance, usually no more than a few inches. Namely, an initiator device uses an antenna to generate an electro-magnetic (EM) field, and a target device which is positioned in the “near-field” area of the generated EM field would use the antenna of the target device (such as a coil) to couple with the antenna of the initiator device, so that data can be transferred via their coupling. Therefore, the NFC is convenient for transmitting/receiving data by simply placing the initiator NFC device close to the target NFC device and more and more mobile devices with integrated NFC antennas are available. For example, users can put their smart phone (having integrated NFC antenna) on or close to a target device and the information of the target device will show on the screen of the smart phone.
However, the location of the antenna integrated within the NFC device depends on different manufacturers and is not standardized. This would cause a problem because the antenna within the initiator NFC device has to be placed right over the antenna of the NFC target device to ensure a proper EM field coupling and therefore a stable data transfer between the devices. Therefore, how to ensure a stable data transmission under the random position of the NFC devices becomes a problem.
One of the approaches is to use a large antenna to cover the whole surface of the target NFC device for proper coupling to the antenna of the initiator NEC device. This solution is not ideal, because two NFC antennas with completely different size lead to a decreased coupling efficiency. Another solution is to use a multi-coil antenna to replace the large antenna mentioned above. As FIG. 1 shows, there are plurality of coils 310 overlapped with one another to form a multi-coil antenna, and each of the coils 310 is connected to a multiplexer 320. The coil in the middle has the best coupling effect with the antenna 331 of the initiator NFC device 330 and is enabled by the multiplexer to communicate with the initiator NFC device 330. The enabled coil is drawn in bold dotted line as shown in FIG. 1. However, this approach still has shortcomings. Namely, if the antenna 331 of the initiator NEC device 330 is placed across more than one coil, and the multiplexer still only enables one coil to communicate with the initiator NEC device 330, it will result in that the antenna 331 of the initiator NFC device 330 could not receive enough power from the enabled coil. Thus, the coupling effect between the only one enabled coil and the antenna 331 of the initiator NFC device 330 is not strong enough to maintain a stable connection. In addition, the electromagnetic power generated by the enabled coil may induce disturbance coupling effects with the nearby metals or other coils, and the disturbance coupling effects may decrease the coupling efficiency between the antenna 331 of the initiator NFC device 330 and the enabled coil of the target NFC device.
Therefore, there still remains another approach to solve the problem of how to build a stable communication between two NFC antennas to ensure a stable data transmitting regardless of the exact positioning of the initiator NFC device.